THE INJURED WORKER & A FELLOW WORKER WERE IN THEPROCESS OF LOOSENING BOLTS HOLDING A CONVEYOR BELT PULLEY IN PLACE. THEY WERE WORKING FROM THE BASKET OF A MANLIFT ON THE OVERHEAD CONVEYOR. WHI LE STRAINING TO BREAK THE RUSTED NUTS FREE, THE WRENCH SLIPED CAUSING THE WORKER TO TWIST HIS BACK, STRAINING SOME MUSCLES.